1. Field of the Invention
Generally, the present invention relates to a golf club shaft and more particularly to an anisotropic golf club shaft which has an improved strength, is produced with a high productivity, and is lightweight.
2. Description of the Related Art
Needless to say, it is advantageous to hit a golf ball straight to get a good score and hit it a long distance. However, many golfers puzzle over the fact that golf balls they hit are likely to be curved, i.e., they hit a so-called hook ball or a so-called slice ball.
The golf ball is curved because the orientation of the orbit of a club head and the orientation (orientation of line normal to face of club head) of the face of the club head are not coincident with each other at an impact time. That is, when the face (orientation of line normal to face of club head) of the club head is directed to the right with respect to the orbit of the club head, the golf ball is curved to the right (slice in the case of right-handed player), whereas when the face of the club head is directed to the left with respect to the orbit of the club head, the golf ball is curved to the left (hook in the case of right-handed player).
Thus, to hit the golf ball straight to an aimed direction, it is necessary to correct the orientation of the face of the club head at an impact time. But it is not easy to correct a swinging form. Thus, many players puzzle over how to correct their swinging forms.
In Japanese Laid-Open Patent Publication No. HEI3-227616, the present applicant describes that in a hollow or solid shaft having an anisotropic material such as fiber reinforced resin or the like formed at at least one part of the shaft, it is possible to differentiate the principal elastic axis of the shaft from the principal geometric axis by differentiating (varying) a fibrous angle of the anisotropic material partly in the circumferential direction of the shaft and at at least one part of the shaft in the thickness direction thereof. In this manner, the principal elastic axis can set at an arbitrary position.
In the hollow shaft in which the principal elastic axis is differentiated from the principal geometric axis, when a load is so applied downward to the shaft that the load does not pass through a point located on the principal elastic axis, the hollow shaft is flexed and twisted, as shown in FIGS. 10 and 11. That is, as shown in FIG. 10, supposing that one end of a hollow shaft 10 is denoted by 10c and that the other end thereof is denoted by a free end 10d, a principal elastic axis E is not coincident with a principal geometric axis G, and the free end 10d is positioned upward from a point Q located on the principal elastic axis E. When a load W not passing through the point Q located on the principal elastic axis E is applied to the free end 10d of the shaft 10, the shaft 10 is flexed and twisted, as shown in FIG. 11.
The present applicant proposed a golf club to which a hollow shaft having the above-described anisotropic property is applied, as disclosed in Japanese Patent Application No. HEI9- 146950. According to the disclosure made therein, the shaft is twisted by the flexure thereof when the golf club is swung so that when a hooker or a slicer uses the golf club, the orientation (orientation of line normal to face of club head) of the face of the club head is self-corrected. In the golf club, the club head is installed on the front end of the anisotropic shaft which is flexed and twisted such that a line normal to the face of the club head is oriented to the direction in which a golf ball is to be flied, i.e., the face of the club head is oriented to a specific direction owing to twisting of the shaft at a desired angle caused by flexure thereof which occurs when the golf club is swung.
In the above Japanese Patent Application No. HEI9- 146950, an anisotropic shaft is manufactured by winding on a mandrel a semicircumference-long prepreg in a region of 0xc2x0xe2x89xa6xcex8 less than 180xc2x0 (first semi-circumference region) and in a region of 180xc2x0xe2x89xa6xcex8 less than 360xc2x0 (second semicircumference region) in the circumferential direction of the shaft, respectively such that reinforcing fibers of both prepregs incline in opposite directions with respect to the axial direction of the shaft. A plurality of one circumference-long layers each consisting of two semicircumference-long prepregs inclining in opposite directions with respect to the axial direction of the shaft is wound one upon another on the mandrel to produce the anisotropic shaft. According to this method, an uncontinuous portion (seam of prepregs: dividing line) of the reinforcing fibers is formed in the boundary between the first and second semicircumference regions of the prepreg corresponding to one circumference of the shaft. Thus, the strength of the shaft is low at the uncontinuous portion. Further, two semicircumference-long prepregs are wound per one circumference of the shaft. Thus, it takes long to manufacture the golf club shaft and further, there may be a variation in the characteristics of products.
In Japanese Laid-Open Patent Publication No. HEI10-71220, there is proposed a golf club shaft which is flexed and twisted (flexed and twisted around the axis) similarly to the golf club shaft disclosed in Japanese Patent Application No. HEI9-146950. According to the disclosure, a shaft is divided into two circular-arcs in a cross-sectional face of the shaft to divide the shaft into two regions along the longitudinal direction (axial direction) of the shaft; and semicircumference-long prepregs are layered one upon another in the two regions such that the fibrous direction, amount of fiber, and resin impregnation amount of a prepreg positioned in one region are different from those of a prepreg positioned in the other region. However, because the semicircumference-long prepregs are layered one upon another in the two regions partitioned along the longitudinal direction (axial direction) of the shaft, an uncontinuous portion (seam of prepregs: dividing line) of the reinforcing fiber is formed in the boundary between the first and second semicircumference regions of the prepreg corresponding to one circumference of the shaft. Consequently, there is a variation in the characteristics of products.
The present applicant proposed a golf club shaft and a method of manufacturing the golf club shaft, as disclosed in Japanese Patent Application No. HEI9-242340 to solve the disadvantage of the anisotropic shaft in which the dividing line is formed on the cross-sectional face of the shaft because two semicircumference-long prepregs are wound per one circumference of the shaft.
In the golf club shaft and the method of manufacturing the golf club shaft, a hoop layer whose reinforcing fiber is substantially perpendicular to the axial direction of the shaft is layered one upon another on the boundary (uncontinuous portion of reinforcing fibers) between the first semicircumference region consisting of one semicircumference-long prepreg whose reinforcing fiber inclines in one direction and the second semicircumference region consisting of the other semicircumference-long prepreg whose reinforcing fiber inclines in the opposite direction. This construction has been devised to prevent deterioration of the strength of the boundary between the first and second semicircumference regions. Two semicircumference-long prepregs whose reinforcing fibers incline in opposite directions with respect to the axis of the shaft are bonded to the hoop layer to prepare a composite prepreg sheet in advance. The composite prepreg sheet is wound on the peripheral surface of a molding core material (mandrel) to manufacture the golf club shaft in a shorter period of time and reduce the degree of variation in the characteristics of products.
In the proposal disclosed in Japanese Patent Application No. HEI9-242340, it is possible to allow the strength and productivity of the shaft to be higher than those of the shaft not provided with the hoop layer. But the shaft has a seam (boundary between two semicircumference-long prepregs) present in one turn of the prepreg wound on one circumference of the shaft. Thus, the strength of the shaft is still low.
It is ideal that the edges of the two prepregs are butted each other at the seam without forming a gap therebetween and without overlapping them on each other. But it is difficult to butt them each other in such an ideal state in factories because they are operated for a mass production. Thus, there is necessarily a variation in the characteristics of products. In other words, in order to accomplish such an ideal butting of the prepregs, it is necessary for skilled operators to work without sparing any effort and time, which lowers the productivity of the shaft greatly. Even though the edges of the two prepregs are butted each other at the seam without forming a gap there between and without overlapping them on each other, the presence of the seam cannot be ignored in view of the durability of the shaft.
In the case of the conventional normal shaft (isotropic shaft: principal elastic axis and principal geometric axis are coincident with each other), in order to allow the shaft to have a uniform property in its circumferential direction, not a prepreg having a length corresponding to a semicircumference of the shaft but a prepreg having a length corresponding to at least one circumference thereof is wound thereon. In the case of the anisotropic shaft, semicircumference-long prepregs are used. Thus, when the same amount of prepreg is used to manufacture the anisotropic shaft and the normal shaft, the total number of prepregs to be used for the former is more than that of prepregs to be used for the latter. Further, in the former, it is necessary to butt two prepregs each other for each one circumference of the shaft, and furthermore, it is troublesome to handle semicircumference-long prepregs. Thus, the productivity of the anisotropic shaft is low.
Needless to say, a lightweight golf club shaft is advantageous for improving the flight distance of a golf ball. It is difficult for most amateur players to swing a heavy shaft at a high head speed and thus difficult to hit a golf ball a long distance except high-level players, professionals, players whose arms have high muscular strengths. Thus, research and development are being made to improve the material of a prepreg and shaft-manufacturing technique to thereby produce a lightweight normal shaft (not having anisotropy). Research and development are also being made to improve an anisotropic shaft which is lightweight and has a high degree of strength and productivity.
The present invention has been made in view of the above-described situation. Thus, it is an object of the present invention to provide an anisotropic golf club shaft which flexes and twists and has a high degree of strength and productivity and is lightweight.
In order to achieve the object, there is provided a golf club shaft comprising a laminate of a plurality of fiber reinforced resinous layers formed of one pair of angle layers consisting of a first angle layer and a second angle layer or a plurality of pairs of angle layers consisting of the first and second angle layers such that a fiber of the first angle layer and that of the second angle layer incline in opposite directions at an angle 20xc2x0-35xc2x0 with respect to an axis of the golf club shaft. The first angle layer and the second angle layer are wound by 1.5 turns in a cross-sectional face of the golf club shaft. A winding start point of the first and that of the second angle layers are dislocated at 180xc2x0 in a circumferential direction of the golf club shaft. In a cross-sectional face of the golf club shaft, a part consisting of two first angle layers and one second angle layer layered one upon another and a part consisting of one first angle layer and two second angle layers layered one upon another are formed to differentiate constructions consisting of the first and second angle layers of both of the parts.
In the construction, by merely winding the first and second angle layers by 1.5 turns, one of the part consisting of two first angle layers and one second angle layer layered one upon another and the part consisting of one first angle layer and two second angle layers layered one upon another is positioned in a first semicircumference region (region of 0xc2x0xe2x89xa6xcex8 less than 180xc2x0) of the shaft in its circumferential direction, and the other of the two parts is positioned in a second semicircumference region (region of 180xc2x0xe2x89xa6xcex8 less than 360xc2x0) thereof. Consequently, the layered construction (orientation state of reinforcing fiber) of the angle layer of the one part and that of the angle layer of the other part are different from each other. Thus, unlike the conventional anisotropic shaft, namely, without winding a plurality of one circumference-long layers each consisting of two semi circumference-long angle layers (prepregs) inclining in opposite directions with respect to the axis of a shaft one upon another, it is possible to flex and twist the shaft and allow the shaft to have anisotropy. Thus, there is not a seam in each angle layer per one circumference of the shaft and thus the anisotropic shaft of the present invention is allowed to have a higher degree of strength than the conventional anisotropic shaft.
Because no semicircumference-long prepreg is used, it is unnecessary to butt edges of the semicircumference-long prepregs each other and hence possible to shorten the manufacturing period of time and improve the productivity.
In the conventional anisotropic shaft in which semicircumference-long prepregs whose reinforcing fibers incline in opposite directions with respect to the axis of the shaft are wound in the first semicircumference region (0xc2x0xe2x89xa6xcex8 less than 180xc2x0) and the second semicircumference region (180xc2x0xe2x89xa6xcex8 less than 360xc2x0) of the shaft in its circumferential direction, the inclination of the reinforcing fiber of the semicircumference-long prepreg with respect to the axis of the shaft is set to 40xc2x0-45xc2x0. This is for the reason described below. That is, the inclination of a reinforcing fiber of a bias layer (layer whose reinforcing fiber inclines with respect to the axis of shaft) formed to allow a normal shaft not having anisotropy to secure a twist rigidity and a twist strength is generally set to 40xc2x0-45xc2x0, which is applied to the above-described inclination of the reinforcing fiber of the semicircumference-long prepreg which imparts anisotropy to the shaft. Unlike the conventional anisotropic shaft, according to the present invention, the inclination (orientation angle) of the reinforcing fiber of the first angle layer and that of the reinforcing fiber of the second angle layer with respect to the axis of the shaft are set to the angle 20xc2x0-35xc2x0. Thus, compared with the case where the inclinations (orientation angle) thereof are set to other angles, the shaft has a greater twist amount when a certain load is applied to the shaft and has a higher degree of bending rigidity. Thus, it is possible to construct the shaft having a smaller number of fiber reinforced resinous layers (prepregs) and having an anisotropy and a higher degree of bending rigidity. Thus, the shaft is allowed to be more lightweight than the conventional shaft.
It is preferable that in the anisotropic shaft of the present invention, a plurality of pairs of angle layers consisting of the first and the second angle layers is formed; that winding start positions of the first angle layers are equivalent to each other in the circumferential direction of the golf club shaft; and that winding start positions of the second angle layers are equivalent to each other in the circumferential direction of the golf club shaft. Consequently, the constructions of the first and second angle layers are equivalent to each other, which prevents an anisotropy displayed by one angle layer from being offset by an anisotropy displayed by the other angle layer. Accordingly, the shaft can display an anisotropy efficiently.
It is preferable that in the anisotropic shaft of the present invention, the fiber reinforced resinous layers include a hoop layer whose fiber inclines at 90xc2x0 with respect to the axis of the golf club shaft and a straight layer whose fiber inclines at 0xc2x0 with respect to the axis of the golf club shaft; that the hoop layer and the straight layer are wound by one turn or integral turns in the circumferential direction of the golf club shaft by dislocating a winding start position of the hoop layer and that of the straight layer by 90xc2x0 from a winding start position of the first angle layer and that of the second angle layer in the circumferential direction of the golf club shaft; and that a dividing line (the winding start position of the first angle layer and that of the second angle layer) is not consistent with the winding start position of the hoop layer and the straight layer. Owing to the construction, the shaft has a higher degree of strength than the conventional anisotropic shaft.
It is preferable that the second angle layer is positioned directly on a periphery of the first angle layer by locating a winding start point of the second angle layer in continuation from a winding termination point of the first angle layer; that the hoop layer is positioned on a periphery of the second angle layer; and that the straight layer is positioned on a periphery of the hoop layer such that the straight layer is positioned on an outermost surface. Owing to the construction, it is possible to allow the part consisting of two first angle layers and one second angle layer layered one upon another and the part consisting of one first angle layer and two second angle layers layered one upon another to be opposed to each other without the two parts being apart from each other a long distance. Thus it is possible to impart an anisotropy to the shaft efficiently.
In the present invention, as the reinforcing fiber of the fiber reinforced resin, it is possible to use a glass fiber, a carbon fiber, various organic fibers, an alumina fiber, a silicon carbide fiber, a metal fiber, fibers consisting of a mixture of these fibers, a woven cloth or a mat. As resin, it is possible to use polyamide, epoxy, polyester, and the like.
It is possible to form the entire golf club shaft of only the fiber reinforced resinous layer. But in addition to the fiber reinforced resinous layer, it is possible to use an anisotropic material layer such as a fiber reinforced rubber layer and a rubber layer having an orientation in combination. In addition, it is possible to form a part of the golf club shaft of resin or rubber not containing fiber.
The angle layer which allows the shaft to flex and twist may be formed partly thereon in its axial direction. That is, the angle layer may be provided on the shaft entirely or partly in its axial direction.